The present invention relates to several methods for efficient production of oxygen by cryogenic air separation. In particular, the present invention relates to cryogenic air separation processes where it is attractive to produce at least a portion of the total oxygen with purity less than 99.5% and, preferably, less than 97%.
There are numerous U.S. patents that teach the efficient production of oxygen with purity less than 99.5%. Two examples are U.S. Pat. Nos. 4,704,148 and 4,936,099.
U.S. Pat. No. 2,753,698 discloses a method for the fractionation of air in which the total air to be separated is prefractionated in the high pressure column of a double rectifier to produce a crude (impure) liquid oxygen (crude LOX) bottoms and a gaseous nitrogen overhead. The so produced crude LOX is expanded to a medium pressure and is completely vaporized by heat exchange with condensing nitrogen. The vaporized crude oxygen is then slightly warmed, expanded against a load of power production and scrubbed in the low pressure column of the double rectifier by the nitrogen condensed within the high pressure column and entered on top of the low pressure column. The bottom of the low pressure column is reboiled with the nitrogen from the high pressure column. This method of providing refrigeration will henceforth be referred to as CGOX expansion. In this patent, no other source of refrigeration is used. Thus, the conventional method of air expansion to the low pressure column is replaced by the proposed CGOX expansion. As a matter of fact, it is cited in this patent that the improvement results because additional air is fed to the high pressure column (as no gaseous air is expanded to the low pressure column) and this results in additional nitrogen reflux being produced from the top of the high pressure column. It is stated that the amount of additional nitrogen reflux is equal to the additional amount of nitrogen in the air that is fed to the high pressure column. An improvement in the efficiency of scrubbing with liquid nitrogen in the upper part of the low pressure column is claimed to overcome the deficiency of boil-up in the lower part of the low pressure column.
U.S. Pat. No. 4,410,343 discloses a process for the production of low purity oxygen which employs a low pressure and a medium pressure column, wherein the bottoms of the low pressure column are reboiled against condensing air and the resultant air is fed into both the medium pressure and low pressure columns.
U.S. Pat. No. 4,704,148 discloses a process utilizing high and low pressure distillation columns for the separation of air to produce low purity oxygen and a waste nitrogen stream. Feed air from the cold end of the main heat exchangers is used to reboil the low pressure distillation column and to vaporize the low purity oxygen product. The heat duty for the column reboil and oxygen product vaporization is supplied by condensing air fractions. In this patent, the air feed is split into three substreams. One of the substreams is totally condensed and used to provide reflux to both the low pressure and high pressure distillation columns. A second substream is partially condensed with the vapor portion of the partially condensed substream being fed to the bottom of the high pressure distillation column and the liquid portion providing reflux to the low pressure distillation column. The third substream is expanded to recover refrigeration and then introduced into the low pressure distillation column as column feed. Additionally, the high pressure column condenser is used as an intermediate reboiler in the low pressure column.
In international patent application #PCT/US87/01665 (U.S. Pat. No. 4,796,431), Erickson teaches a method of withdrawing a nitrogen stream from the high pressure column, partially expanding this nitrogen to an intermediate pressure and then condensing it by heat exchange against either crude LOX from the bottom of the high pressure column or a liquid from an intermediate height of the low pressure column. This method of refrigeration will now be referred to as nitrogen expansion followed by condensation (NEC). Generally, NEC provides the total refrigeration need of the cold box. Erickson teaches that only in those applications where NEC alone is unable to provide the refrigeration need that supplemental refrigeration is provided through the expansion of some feed air. However, use of this supplemental refrigeration to reduce energy consumption is not taught. This supplemental refrigeration is taught in the context of a flowsheet where other modifications to the flowsheets were done to reduce the supply air pressure. This reduced the pressure of the nitrogen to the expander and therefore the amount of refrigeration available from NEC.
In U.S. Pat. No. 4,936,099, Woodward et al use CGOX expansion in conjunction with the production of low purity oxygen. In this case, gaseous oxygen product is produced by vaporizing liquid oxygen from the bottom of the low pressure column by heat exchange against a portion of the feed air.
In DE-28 54 508, a portion of the air feed at the high pressure column, pressure is further compressed at the warm level by using work energy from the expander providing refrigeration to the cold box. This further compressed air stream is then partially cooled and expanded in the same expander that drives the compressor. In this scheme, the fraction of the feed air stream which is further compressed and then expanded for refrigeration is the same. As a result, for a given fraction of the feed air, more refrigeration is produced in the cold box. The patent teaches two methods to exploit this excess refrigeration: (i) to produce more liquid products from the cold box; (ii) to reduce flow through the compressor and the expander and thereby increase flow to the high pressure column. It is claimed that an increased flow to the high pressure column would result in a greater product yield from the cold box.
In U.S. Pat. No. 5,309,721, the low pressure column of a double column process is operated at a pressure much higher than the atmospheric pressure. The resulting nitrogen stream from the top of the low pressure column is divided into two streams and each stream is expanded in a different expander operating at different temperature levels.
The U.S. Pat. No. 5,146,756 also teaches the use of two expanders to obtain large temperature differences between the cooling and warming streams in the main heat exchanger that cools the feed air stream for distillation. This is done to reduce the number of main heat exchanger cores. However, in order to operate two expanders, the low pressure column is run at pressures greater than 2.5 bar and a portion of the nitrogen exiting from the top of the low pressure column is expanded in one of the expanders. A portion of the feed air is expanded in the second expander to the low pressure column.
U.S. Pat. No. 4,543,115 teaches a double column process where two different pressure feed air streams are produced by compression and fed to the cold box for separation. The lower pressure air stream is fed to the low pressure distillation column, while the higher pressure air stream is sent to the high pressure column. The double column process produces low purity oxygen and nitrogen products.
U.S. Pat. No. 4,964,901 also teaches the use of two pressure air feeds to the cold box for separation. The lower pressure air stream is about 1.5 to 1.8 bar pressure and is withdrawn from an interstage of the main air compressor. The rest of the air is further compressed to a higher pressure and sent to the high pressure column. The lower pressure air is sent to the low pressure column. The problem with such a process is that a separate adsorbent bed is to be used to remove impurities such as water and carbon dioxide from the lower pressure air stream. Due to the lower pressure, a large quantity of water is present in the lower pressure air stream and this not only increases the size of the adsorbent beds but also the energy required for the regeneration of these beds. This leads to an expensive process.